Recently, development of ITSs has been underway aiming at reduction in traffic accidents. One type of the ITSs includes wireless base stations (roadside devices) installed in or on traffic lights for roadside-vehicle communication wherein information, such as traffic light information at an intersection (e.g., whether the light is red or green) or whether there is any vehicle turning to the right is sent to a wireless terminal (hereinafter referred to as a “on-vehicle device” or “mobile station”) mounted on a vehicle that is a few hundred meters from that intersection. In addition, there is another type of inter-vehicle communication in which on-vehicle devices that are separated with a smaller distance inform each other of vehicle information, such as the respective travel direction and traveling speed.
For such inter-vehicle communication, application of narrow-area ad-hoc communication based on the common wireless Local Area Network (LAN) techniques (IEEE802.11 series standards), as disclosed in Non-Patent Document 1 listed below is considered. The narrow-area ad-hoc communication in accordance with Non-Patent Document 1 is a type of communication in which mobile stations can directly communicate with each other without requiring any central control apparatus, such as a base station. Since no base stations are involved with allocating communication (wireless) resources, each mobile station manages communication resources that are employed, and carry out communication while contending each other for communication resources (hereinafter, referred to as shared resources) shared among mobile stations in a decentralized autonomous manner.
That is, each wireless terminal sends signals while monitoring a state of usage of the shared resources (frequency) by using the Carrier Sense Multiple Access (CSMA) technique so that the sending signals do not collide with each other. Furthermore, each wireless terminal waits for receiving data while it is not sending, and searches for beginnings (preambles) of signals being sent by other wireless terminals. This means that wireless terminals are not synchronized with each other. Note that IEEE 802.11 series standards indicate that cells having radii within several hundred meters are constructed as communication areas and the Orthogonal Frequency Division Multiplexing (OFDM) is employed as a modulation scheme.
Note that well-known techniques related to allocating communication resources include techniques described in Patent Documents 1 and 2 listed below.
The technique of Patent Document 1 is related to a packet relaying scheme in a wireless transmission. It is directed to providing a scheme in which reliability as a system is ensured for failure of an apparatus during relay while requiring only a single carrier for conveying wireless signals, in terms of the relaying scheme of wireless signals applied when a distance between locations of a number of sites distributed and a center for collecting information is beyond the limit of the propagation distance of wireless signals.
For this purpose, Patent Document 1 discloses that a plurality of portable terminals that transmit sensor information and monitor control information in a packet to a main terminal are linked via a linear transmission path. A time slot related to an installation location is allocated to each portable terminal, thereby permitting wireless transmission in this time span for relaying packets among portable terminals in a time division multiplexing manner as bucket brigade while relaying by bypassing an adjacent portable terminal when this terminal fails.
In addition, the technique of Patent Document 2 relates to a method for allocating wireless channels in a wireless communication system and a corresponding wireless communication system. It is directed to providing an improved method and wireless communication system for allocating wireless channels.
For this purpose, the Patent Document 2 discloses that a wireless channel in the downlink direction (tsx) is allocated to a mobile station (MS) by a wireless station (BS) depending on a channel allocation pattern (CAP) that indicates a transmission output (txp) for a time slot, and corrects the allocation of the wireless channel (tsx) by mobile station (MS) under the control of the mobile station depending on an interference pattern (IP) that indicates an interference output (inp) of the time slot.
As described above, in the technique of Patent Document 2, by carrying out allocating depending on the occupy rate of the time slots according to channel allocation patterns, it is possible to distribute the transmission output from a wireless station in the downlink direction as evenly as possible among existing time slots. By combining this feature and the fact that correction of such allocated time slots under the control of mobile stations is available (that is, correction to a time slot having a lower interference output based on the interference pattern), the shortcoming of soft handover may be avoided in the case in which the frequency repetition interval (cluster reuse) is equal to one and interferences may be problematic.
Patent Document 1: Japanese Patent Publication No. H09-36916
Patent Document 2: Japanese Translation of PCT International Application No. 2002-510916
Non-Patent Document 1: IEEE 802.11, 1999 Edition
However, none of Patent Documents teaches control on communications between mobile stations.
The related techniques described above may be problematic when the number of mobile stations is increased and thus there is not sufficient resources used for communications between mobile stations.